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Propulsion - Many Roads Go Up

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Propulsion - Many Roads Go Up PROPULSION - MANY ROADS GO UP zrt - IC7 HE TECHNOLOGICAL revolution of the past wings, others derive their lift from their body geom- decade and a half has left its imprint on almost etry, others are symmetrical and will produce little or Tevery facet of air and space vehicle development. no lift. Regardless of their mission the performance Nowhere is this more dramatically illustrated than in of each of these vehicles will depend in high degree the vital field of propulsion—key to range, payload, on the effectiveness of its propulsion system. and speed. Many improvements in existing engine schemes can Technical progress since World War II has opened be expected, some of them of a radical nature. It is up innumerable types of possible engine systems com- also possible that, in the decade ahead, some funda- pared to the few that were feasible in the early 1940s. mentally new means of propulsion will appear. At that time reciprocating engines were the prin- Development costs are extremely high. The power cipal type in development. Turbojets, ramjets, and plant normally is the principal expenditure item for rocket engines were just emerging from the labora- any weapon system, and its development can run to tories of a few persistent pioneers. several hundred million dollars. If a major basic- or Today the Air Force is investigating or develop- applied-research program is involved, the total cost ing turbojets, ramjets, liquid-fuel rockets, solid-fuel of obtaining a satisfactory engine can go to a billion rockets, nuclear rockets, nuclear ramjets, nuclear tur- dollars or more. bojets, ion rockets, and plasma rockets. Specific engines Management skill is a key factor, perhaps the most within these general classes include: turborockets, important requirement for orderly and efficient pro- turboramjets, external-burning ramjets, turbofans, lift pulsion programs of the future. The lead times for fans for VTOL use, hybrid rockets using liquid new engines are so long, the needs for men, facilities, and solid propellants, spherical-shaped solid rockets, and dollars are so large, and the technical choices so rockets with plug nozzles, clustered rockets, seg- diverse that there is small room for major error or mented rockets, direct-cycle nuclear turbojets and omission in the space age. The importance of basic ramjets, and indirect-cycle nuclear turbojets and ram- decisions and the long lead times required to repair jets—among others. errors of judgment have been illustrated dramatically The uses to which these engines can be put are as by the construction of "small" engines for US ICBMs. varied as the engines themselves. Many types of recon- These engines perform the ICBM task efficiently but naissance and bombardment vehicles, for use both in do not meet the corollary requirement for large space space and in the atmosphere, are being studied by the boosters. As a result, the US space effort faces a Air Force. Some of these vehicles have recognizable severe payload handicap at least until the mid-1960s. 48 AIR FORCE Magazine • February 1961 Technological progress in the field of aerospace propulsion presents stirring new vistas and weighty new problems. Upcoming decisions in this vital area will have a considerable effect on the nation's military strength, space capabilities, and budgets in the years ahead ... J. S. Butz, Jr. TECHNICAL EDITOR, AIR FORCE MAGAZINE PART I — THE AIR-BREATHERS The extreme payload sensitivity of its booster sys- cult. It will be impossible to develop and operate all tem is the primary reason for the stretchout in design of the technically promising propulsion systems and and development of Project Mercury. The US is vehicles. Many feasible systems must be bypassed. The limited to a total capsule weight of about one ton only certainty is that this sorting-out process and the for its first man-in-space program; the Soviet Union search for the best means of using available resources apparently has a five-ton leeway for the same mis- is certain to grow more difficult with time. sion. Undoubtedly, keeping weight down will become The multitude of possible propulsion systems may a problem with Dyna-Soar. All new types of vehicles be broken down into five general classes to allow a seem to gain weight beyond their original estimates. closer look at some of the technical decisions which The magnitude of the management task for all will have to be made in the near future and during vehicle systems is certain to bring changes in devel- the 1960s. opment organization and procedures. This has been These decisions will have a considerable effect on the theme of suggestions in every critique of the US the military posture of this country, its ability to ex- research and development effort, from Von Karman's plore and control space in the 1970s, and on the na- Toward New Horizons of 1944 to the Stever report of tional budgets. 1959. The five classes of engines are: air-breathers, rockets, The three main suggestions are: ( 1) longer term electric systems, propellant-collection systems, and planning and financing to cover several years or at nuclear and advanced systems. least the complete life of a development project; ( 2 ) The ultimate potential for air-breathing engines does increased specialization and technical competence for not yet appear to be in sight in any speed range and officers in R&D posts which require major changes in for any type of aircraft. Improved designs for light- current personnel policies; (3 ) greater authority and weight VTOL lifting engines and Mach 3 turbofans freedom of action for contractors and military per- are now under development by the military and US sonnel on the working level of research and develop- engine manufacturers. Air-breathing engines which ment programs. Under this arrangement highly placed will operate up to escape speeds of 25,000 mph at commanders and civil authorities will retain close con- altitudes of 250,000 feet or better are now considered trol only over general policy and the selection of major possible by competent engine designers, although programs. there is little experimental proof to back up their Regardless of organizational improvements the main theoretical predictions. Several types of air-breathing task of top management will remain extremely diffi- (Continued on following page) AIR FORCE Magazine • February 1961 49 PROPULSION CONTINUED COMBUSTION CHAMBER NOZZLE INLET SHOCK WAVES engines are considered possible at these extreme speeds and altitudes, including turborockets, super- AM sonic combustion ramjets, and external-burning ram- jets (see accompanying cuts). However, the course of air-breathing engine devel- ■IIIIIMIMIMIIIIIIIIIIIIIImmimmNm..... opment above Mach 6 or so already has been greatly influenced by management decisions. The National Aeronautics and Space Administration essentially CONVENTIIONAL SUBSONIC COMBUSTION RAMJET eliminated air-breathing engine research from its pro- gram shortly after it was formed around the nucleus of the personnel and facilities of the National Ad- COMBUSTION CHAMBER visory Committee for Aeronautics. The efforts of the - NOZZLE \INLET SHOCK WAVES . group at Lewis Research Center, Cleveland, Ohio, one of the world's largest and most experienced air-breath- V ing engine research teams, was redirected at that time. This was a controversial decision, both within NASA and among power-plant experts throughout the entire western world. The reasoning behind the decision was 111111111■ that no further basic or applied research was needed 111........miiiiiiiiiiiiiiiiiiim.......... by manufacturers to build a new generation of im- proved turbojets for speeds below Mach 4. This essen- SUPERSONIC COMBUSTION RAMJET tially has proved to be true because several manufac- turers are working in this area, although much of their effort has been of a research rather than a develop- mental nature, and NASA data could have been helpful. INLET SHOCK WAVES ...,y, MOVABLE COWL In the hypersonic engine area, it was decided that I experimental research should be dropped and all effort concentrated on a wide variety of rocket engines, from more or less conventional chemical engines to nuclear and electric rockets. It was believed that the primary requirement in the 1960s would be for rockets. Now it appears that the military will not follow '14411111111 PLUG NOZZLE K ' this policy lead and that perhaps NASA will reverse itself and return to experimental research with hyper- • HYPERSONIC COWL-TYPE RAMJET WITH SUPERSONIC COMBUSTION sonic air-breathing engines. The Air Force is planning to begin applied research leading to a large, one- stage, winged Aerospace Plane that can fly into orbit using air-breathing engines rather than rockets. NASA COMPRESSOR WHEEL /- TURBINE WHEEL apparently is going to reenter this propulsion field even PITOT INLET / SHAFT NOZZLE \ , if most of the research work has to be done under con- - --- III --.- '' ,-1-----' ;I/WM: tract,with Lewis Research Center furnishing the man- agement personnel. I i --- 1 ■1 -L--■ ANII.. FUEL INJECTOR- ROCKET CHAMBER ' Schematic drawings of five types of air-breathing engines COMBUSTION CHAMBER which have potential usefulness at hypersonic speeds are shown at left. The air flow inside the "conventional" ram- jet at the top is slowed down to subsonic speeds. This type of engine probably will not be feasible above Mach TURBOROCKET ENGINE 6 or 7 because its internal temperatures will be so high that cooling will be impractical with known methods. The air flow in the ramjet second from the top is supersonic in all portions of the engine except the nozzle throat. WING OR UNDER SURFACE This results in a cooler-running engine than the "conven- OF FUSELAGE tional" ramjet. Further research will be required to prove \ \ that stable combustion can be maintained in a supersonic \ \ rDIRECTION ' - FUEL INJECTION airstream. The middle drawing shows a type of ramjet that POINT \ OF FLOW \ will operate up to speeds of 18,000 mph, according to the ---1.-.
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